Gun Barrel Optimized for Supersonic Projectiles

ABSTRACT

This patent is for an apparatus which increases the muzzle velocity and energy of a supersonic projectile for a given set of ballistic conditions. When a projectile is fired from a barrel, there is typically a great deal of available energy still remaining in the combustion gas. This energy is released from the muzzle without being used when the projectile exits. Friction between the barrel and projectile, and the length, cost, and weight of the barrel limit the amount of energy which can be transferred into the projectile. This apparatus may constitute an attachment to the end of a pistol, rifle, or other gun; it may be a modification to the barrel of a gun; or it may be incorporated into an entirely new barrel for a gun. 
     The apparatus consists of a non-contact section, and a diverging section followed by a straight, constant area section, and optionally followed by a short converging section. The apparatus works by providing the optimal geometry for the air in front of the projectile to exceed sonic velocity. The sonic shock wave is then moved out in front of the device, which reduces the air pressure at the front of the projectile and increases its acceleration. Additionally, as the projectile enters the device it loses physical contact with the walls so friction is eliminated. With a supersonic projectile, the combustion gas behind it must travel at supersonic speed to move around and past the projectile and this produces very large wave drag. So even though there is not a tight seal between the projectile and the wall, the expanding gas still exerts considerable accelerating force on the projectile. Also, the gas that does flow around the projectile produces a hypersonic jet of air exiting the device ahead of the projectile, in effect blazing a trail through the air for the projectile to follow. 
     Furthermore, in the non-contact section the wall friction is negligible while the fluid friction is significant which causes the projectile to absorb more available energy from the propellant.

BRIEF DESCRIPTION

Every rifle barrel currently made is constructed on the assumption that it is necessary to maintain a tight fit between the projectile and the bore of the barrel along its entire length, in order to minimize blowby and transfer the maximum amount of energy to the projectile. That assumption is valid only for subsonic projectiles; it is invalid when the projectile attains sonic speed. Failure to properly account for supersonic effects limits the performance which can be achieved. Consequently, a typical high powered rifle will transfer only about 10% of the energy released by the burning propellant into the projectile.

This invention is for a rifle barrel that is designed to utilize the phenomenon of supersonic fluid flow to achieve better performance. The construction of the barrel is divided into a subsonic region and a supersonic region. The subsonic region is identical to a conventional rifled barrel, and is a relatively short portion of the total length. The supersonic region is the novelty claimed in this patent.

When the projectile enters the supersonic region, it no longer physically contacts the bore of the barrel. The projectile is then aerodynamically supported and guided down the barrel by the boundary layer. This drastically reduces the friction force on the projectile, eliminates wear of this portion of the barrel, and enables the usage of new materials and/or surface treatments to reduce heat transfer into the barrel. Additionally the end of the barrel is designed to eliminate the hemispherical shock wave at the muzzle and produce a supersonic jet of gas in front of the projectile. The supersonic jet will usually be underexpanded, and consequently will form a diamond pattern of oblique shock waves which will have minimal effect on the stability of the projectile. This invention can be applied to every type of gun, launcher, or device which propels a projectile at supersonic velocity.

The profile of the barrel is generally as shown in Sketch A and Sketch B. The barrel is manufactured with standard machining processes whereby the bore of the subsonic section is drilled and rifled in the normal way. The supersonic section is drilled to a slightly larger diameter, as calculated according to the ballistic characteristics of the gun, and the diverging section is bored, drilled or reamed to achieve the desired shape.

DETAILED DESCRIPTION

As a projectile accelerates down the barrel it pushes the air in front of it out the muzzle. If the bore has a constant cross sectional area along its length, and the flow is subsonic, then the velocity of the air exiting the muzzle will be nearly identical to the velocity of the projectile. The kinetic energy of the air is dissipated when it exits, known as exit loss, but this is minimal and of little importance. However, when sonic speed is reached, the air speed exiting the muzzle is limited to sonic speed, which is less than the speed of the projectile. Consequently the air pressure increases in front of the projectile and impedes its acceleration. Also, a strong shock wave forms at the muzzle, normal to the flow of air, causing the energy in the air to be mostly converted to heat.

In this invention, the end of the barrel is designed as a special diverging nozzle. At subsonic projectile speeds, the kinetic energy of the air in front of the projectile is isentropically converted to pressure. Because the pressure outside of the barrel is constant at atmospheric pressure, the air pressure inside the barrel is reduced to sub-atmospheric pressure. This effect causes a significant portion of the air in front of the projectile to be evacuated before sonic speed is achieved. When sonic speed is reached, then the pressure in front of the projectile rises just as it does in a conventional design. However, because the pressure is lower when sonic speed is reached with this invention, the pressure rises to a lower level than it otherwise would. The net result of this is that less work is done on the exiting air, which essentially gets wasted and slightly more energy is retained in the projectile. However, the energy in the exiting air is not completely wasted, because it accelerates the air in front of the projectile and reduces its drag to a small extent. The table below shows how the pressure at the throat, the place at which the diverging section begins, decreases as the projectile accelerates toward Mach 1. The air pressure in the region between the nose of the projectile and the throat will be nearly the same as at the throat.

Mol. Wt. 28.8 area ratio 2.9 exit pressure 14.5 psia Mach # p-throat Speed-bullet Speed-exit air velocity pressure 0.1739 14.21 200 69.0 0.288 0.2608 13.86 300 103.4 0.630 0.3478 13.42 400 137.9 1.079 0.4347 12.89 500 172.4 1.609 0.5217 12.30 600 206.9 2.195 0.6086 11.68 700 241.4 2.813 0.6956 11.05 800 275.9 3.441 0.7826 10.43 900 310.3 4.063 0.8695 9.833 1000 344.8 4.667 0.9565 9.256 1100 379.3 5.243

As the projectile accelerates through the subsonic portion of the barrel it behaves identically as in a conventional design. But when it enters the supersonic section, its behavior will be controlled by fluid dynamic phenomenon instead of physical contact with the bore. There is a very small clearance between the projectile and the bore, in which exists the fluid boundary layer. For the high pressure gas behind the projectile to escape through this clearance space it would have to travel through the boundary layer at supersonic speed relative to the surface of the bore. This does not happen. The supersonic portion begins with a constant diameter section through which the projectile accelerates while being guided, supported and stabilized by the boundary layer. As the projectile travels through this aerodynamically stabilized region, it is largely immune to vibration and slight deflection of the barrel which could affect its accuracy. This means that the use of very heavy barrels, bedding the barrel in the stock, and other measures to eliminate vibration will be largely unnecessary.

Gas lubricated bearings are widely applied to support light loads with extremely low friction. The supersonic region of the barrel relies on the same aerodynamic principals, and the film thickness, boundary layer effects, and other factors which dictate the performance are calculated using the well established procedures for gas lubricated bearings.

The projectile then enters the diverging section. As the clearance between the projectile and bore gradually increases, the boundary layer is less effective at preventing blowby. A portion of the high pressure gas now moves past the projectile. It should be noted that while this gas is moving supersonically with respect to the bore, it is moving subsonically with respect to the projectile. The space between the projectile and bore effectively forms a converging diverging nozzle, which causes the leakage gas to accelerate to a high Mach number. The flow of leakage gas gradually increases as the projectile moves through the diverging section. When the projectile finally exits the muzzle, it is contained within a supersonic jet of gas which is moving forward faster than itself. Aerodynamic drag then continues to accelerate the projectile for some distance downstream of the muzzle. Eventually the gas jet around the projectile dissipates and the projectile crosses a weak shock into a supersonic regime.

This is contrasted to a conventional barrel design. When a supersonic projectile exits the muzzle of a conventional barrel it must penetrate the strong shock wave. At nearly the same time, the high pressure gas contained behind the projectile is instantly released when the projectile exits. The gas expands uncontrollably, and any slight imperfection at the muzzle causes asymmetic forces on the tail of the projectile, tending to disturb its stability. This is a well known effect and great care is taken to preserve circularity of the muzzle bore. After exiting the muzzle the bullet travels some distance before it regains stability, all the while travelling in a supersonic regime with comparatively high drag.

The straight, aerodynamically guided zone enables the projectile to accelerate to higher velocities because friction is drastically reduced, and wear is eliminated so that alternative materials can be used. While the special diverging section recovers velocity energy from the escaping air, controls the expansion of the high pressure propulsion gas, prevents formation of a normal shock wave, and produces a supersonic jet ahead of the projectile.

Description of Sketch A

The device shown in Figure A is an add-on to a short rifled barrel. In this configuration the non-contact section is removable and provides several significant benefits. The heavy, expensive, wear resistant alloy material is limited to a comparatively short section. An existing firearm, perhaps one that has a bent barrel or damaged muzzle, can be remachined and retrofitted with this device. Or this can be fitted to the end of an existing barrel to increase the velocity and energy of the projectile. Alternatively, a firearm can be manufactured specifically to take advantage of this. High powered rifles need a barrel which is long enough to provide for optimal expansion of the combustion gas to achieve the desired projectile velocity. These long barrels make those firearms expensive to manufacture and difficult to carry. Using this device, the contact portion of the barrel will be roughly ½ the normal length, and the non-contact portion roughly ½. A rifle barrel which would ordinarily be 26″ long can now be reduced to a 13″ fixed length with a 13″ detachable length. This will result in greatly reduced manufacturing cost, improved carrying ability, and when the non-contact section is made of a material such as titanium or aircraft-grade aluminum, significantly reduced weight. All or a portion of the non-contact section can be detached while transporting the rifle and then reattached before shooting. Or the detachable end section may be left off for short range, rapid shooting to give good maneuverability, and attached for long range, precision shooting.

Description of Sketch B

The device shown in Figure B is a special section which has been integrally formed into a single piece barrel. This configuration is useful where portability or carryability, is not a concern, but there may be a desire to reduce weight, cost, or angular moment of inertia. The heavy wear resistant alloy is needed in the short contact zone, while a lighter section can be used in the non-contact zone. The contact zone is limited to the length required for the projectile to attain supersonic speed. In the supersonic, non-contact zone, the material is selected for the best strength, cost, or other properties. The rifling in the contact zone is faster, in other words it makes more revolutions per length, so that the projectile achieves the correct angular momentum before entering the non-contact zone. This device is shown with a thermal barrier coating (TBC) in the non-contact zone. The TBC reduces transmission of heat from the gas into the barrel, which keeps more energy available in the gas and also reduces the barrel temperature. 

The claims for this patent are:
 1. an apparatus through which a projectile is accelerated, which does not have constant cross section throughout its length;
 2. where the apparatus in 1 increases the projectile velocity, improves the accuracy, reduces the barrel weight, reduces the barrels angular moment of inertia, reduces the barrel cost, improves the barrel lifetime, or any combination of these;
 3. the apparatus in 1 may be either an added on device to an existing gun barrel or may be a section specially machined or constructed into the end of a gun barrel;
 4. the apparatus in 1 may be constructed either during the original manufacture of the barrel, or may be a later modification to an existing barrel;
 5. where the change in cross sectional area of the apparatus in 1, separates the barrel length into a contact zone and a non-contact zone;
 6. where the change in cross sectional area is designed to accelerate the gas exiting the muzzle to supersonic speed;
 7. where the supersonic gas flow in 6 reduces the aerodynamic drag experienced by the projectile at the end of the barrel and the beginning portion of its free flight.
 8. where the absence of friction in the non-contact zone of 5, and the reduced air resistance of 7 cause the projectile to achieve higher velocity;
 9. where the material of construction for the non-contact zone may be different from the contact zone, and is selected to reduce weight, reduce cost, increase stiffness, or improve the thermal behavior compared to the material used in the contact zone.
 10. the device has a diverging section designed to reduce the air pressure in front of the projectile while it is still in the barrel; a. the non-contact zone and diverging section may be any fraction of the total length of the device, from very small to very large b. the non-contact zone and diverging section may be designed to optimize projectile velocity, projectile stability, weight of the device, manufacturing cost, or any other variable
 11. the device has a straight section of essentially constant area through which the projectile travels at hypersonic velocity, without contacting the walls a. the boundary layer in the straight section provides aerodynamic support to the projectile, stabilizing its path without the projectile actually contacting the wall surface. b. the straight section may contain surface features designed to control the boundary layer which may include: co-rotational or counter-rotational rifling, labrynth seals, hatching, circumferential grooves or ridges, longitudinal grooves or ridges, any other feature which is shown to assist with achieving claim 1, 2, 6 or
 8. 12. the device may have an optional converging section at the end which reduces the gas velocity to near the projectile velocity (because the gas is moving supersonically a converging section reduces speed and increases pressure);
 13. the dimensions of the device are calculated and optimized for a specific gun, projectile, and other ballistic properties. The dimensions may be optimized using computational fluid dynamics (CFD), by trial and error, by hand calculations or other procedure.
 14. the device produces a hypersonic jet of gas in front of the projectile which moves the shock wave to some distance in front of the muzzle or which reshapes the shock wave to minimize effects on the projectiles velocity when it exits the barrel, or which minimizes the imbalanced forces which tend to destabilize the projectile;
 15. the materials of construction in 9 may be metal, polymer, composite, ceramic or some combination of materials a. the material may be selected to improve heat dissipation from the barrel b. the material may be coated with a thermal barrier coating to reduce cooling of the propulsion gasses; c. the material in the non-contact zone need not be wear resistant, and therefore may reduce the cost or weight of the barrel assembly; d. the material may have better machinability, lower cost, better strength/weight ratio, or be compatible with low cost manufacturing methods such as drawing, rolling, sintering, or injection molding.
 16. the device may be integral to the barrel or attached to the barrel by threads, set screws, friction fit, adhesive, magnets, tape, or any other means and may be permanently attached or removable.
 17. the device may have the additional benefits of reducing recoil, report, muzzle flash, or other objectionable side effect of firing the gun. 